NO348132B1 - Underwater depth-compensable accumulator system - Google Patents

Description

Accumulator system

Technical field of the invention

The invention relates to an underwater-based, depth-compensable accumulator system which is designed to generate hydraulic power for underwater operations, and which consists of one or more gas-free accumulator units. An accumulator unit according to the invention has only one moving part, but a cooperation with a switching device enables switching between a large number of modes so that one and the same accumulator unit can be configured to generate a desired hydraulic pressure at any sea depth.

Background for the invention

Future oil extraction will take place from installations at ever greater sea depths, and a prerequisite for the safe operation of such installations is that you have almost immediate access to a significant amount of hydraulic power at all times. Compared to hydraulic systems based on compressed gas, an accumulator system according to the invention will be able to deliver significantly larger amounts of hydraulic power in relation to weight/size. A significant operational advantage is also achieved in that the system can be configured with simple operations for a radical change of depth without having to be pulled up to the surface.

An accumulator system according to the invention is particularly suitable for generating hydraulic energy for the operation of "work-over completion systems". These systems are used for the overhaul of oil wells that can be located at any sea depth.

Known technique

Various valve devices and actuators used in underwater oil operations are largely operated using hydraulic pressure, as the equipment is designed to utilize the overpressure of the hydraulic fluid relative to ambient pressure. The hydraulic power is today preferably produced by the pressure in a compressed gas being transferred to the hydraulic fluid via a displaceable piston which acts as a barrier between the compressed gas and the hydraulic fluid. Gas compressed to very high pressures loses pressure quickly during expansion, and consequently has a limited ability to transfer energy. This ability is further degraded by thermal effects which create a temperature drop which results in a reduction in the transfer pressure.

In order to get more energy out of gas accumulators, depth-compensated accumulators have been developed where care has been taken to establish forces that cancel out the effect of a change in ambient pressure. This is achieved by means of a piston arrangement where two oppositely directed surfaces sense respectively ambient pressure or an approximately zero pressure in a gas cylinder. This solution requires expensive machining, and will not prevent the gas pressure from reaching a pressure level where its compressibility is significantly reduced compared to an ideal gas. This is because some tools require a hydraulic pressure of 345 bar and higher.

NO 20190053 A1, WO 2018/160071 A1, WO 2016/133400 A1 and US 2013/0074687 A1 refer to known technology that is relevant in relation to the invention in question. This technology is based on accumulators that generate hydraulic power by means of displaceable pistons and exploit to varying degrees the pressure difference between ambient water pressure and liquid-free, low-pressure chambers. NO 20190053 A1, WO 2018/160071 is, like the invention in question, based on the fact that energy is stored by establishing an approximately gas- and liquid-free volume in tanks or accumulators at depth, and that energy stored in this way is transformed into hydraulic energy by utilizing the pressure of liquid which is returned to this volume via pressure-sensitive piston devices.

NO20190053 has the greatest similarity with the invention in question in that it is based on completely gas-free accumulators in cooperation with pressure stabilizing valves which ensure that the accumulators can be set for a fixed hydraulic pressure which will be maintained even if the accumulators are moved to a significantly changed depth.

A structural feature that particularly distinguishes the invention in question from prior art is that it comprises a switching device which is designed to cooperate with a displaceable piston device so that various different operating modes are established, each of which represents a changed relationship between the pressure of the driving medium and the hydraulic pressure produced by the accumulator units . The switching device in question can in this way be switched between a number of operating modes, and as a result of this, an accumulator system can be assembled from accumulator units that can individually utilize the energy in a supplied drive medium efficiently to deliver hydraulic energy that maintains a preset pressure level at any sea depth .

Brief description of the drawings

The operation of the accumulator units and an accumulator system according to the invention is described in the following with reference to figures 1 - 6, where

- Fig.1 shows a principle price for a simplified first embodiment of an accumulator system according to the invention.

- Fig.2 shows a principle price for an accumulator unit consisting of three separate chambers

- Fig 3 shows a principle price for a 6-chamber accumulator unit with a simple switching device

- Fig.4 shows an outline of relevant designs of the piston device

- Fig.5 shows a diagram of a preferred embodiment of a pressure stabilizing valve

- Fig.6 shows an almost complete accumulator system according to the invention

Detailed description of the invention

Subsea based hydraulic equipment has a nominal operating pressure which can typically be 345 bar above ambient water pressure. The accumulator system in question can be used to generate any relevant hydraulic pressure, but for the sake of simplicity, the further description is based on 345 bar delivery pressure.

Fig.1 shows a sketch of a simplified hydraulic system which includes an accumulator unit with 6 internal chambers. The propellant is stored in a flexible reservoir 14) and thereby has approximately the same pressure as the surrounding water. The accumulator unit consists of an accumulator housing 1) with an axially displaceable piston device 2-4) which separates 6 chambers (I–VI) with separate connections to 6 ports 5-10) in the accumulator housing. Chamber I is in contact with the piston arrangement's largest pressure surface, and is referred to as the drive chamber, as this chamber is in permanent open connection with the drive medium via the inlet port 5). In principle, the drive chamber is the only chamber which, regardless of the operating mode, is in open connection with the drive medium. Of the other five chambers, at least one is made virtually depressurised, while the rest are filled with liquid and are set in open connection with either the inlet port 5) or the outlet port 11) which is arranged on the aforementioned switching device 12).

The accumulator system will normally comprise a pressure stabilizing valve 20) which is arranged between said reservoir and the accumulator unit(s), and is designed to regulate the supply of drive medium to the accumulator unit(s) so that the hydraulic pressure maintains the desired level regardless of mode. Such a pressure stabilizing valve can be omitted if the accumulator unit is to be used within a limited depth interval, as with the use of 6 chambers it will be possible to keep the hydraulic pressure within, for example, 345 bar /- 10%. The operation of a preferred embodiment of a pressure stabilizing valve will be explained later with reference to fig. 5.

The hydraulic system will normally include a charging device 21) for maintaining hydraulic capacity. In the preferred embodiment, this comprises an electrically driven low-pressure pump and a pressure-boosting pump. The operation of the charging device will be explained later with reference to fig. 6.

How the gas-free accumulator works

The operation of a gas-free accumulator unit is explained with reference to fig. 2 which shows a diagram of an accumulator unit with a piston device 3,4) which separates three separate chambers (I, II, VI) in the accumulator housing 1). The housing has an inlet port 5) for supplying a driving medium, and an outlet port 11) for releasing pressurized hydraulic fluid. The piston device is composed of a piston rod 3) with cross-section A2 and a piston 4) with pressure surface A1, and cooperates with two sliding seals 2,5) which prevent leakage between the three chambers (I, II, VI).

Chamber I is referred to as the drive chamber, and has contact with the piston device's largest pressure surface and with the inlet port 5). The propellant is fed into the propellant chamber I with a pressure PD ≤ PAMB, where PAMB is defined as ambient water pressure. The drive medium will consequently create a force FS = PD*A1 against the piston device. This force seeks to push the piston device in a direction away from the inlet 5), and generates correspondingly large counterforces in chambers I and II. The counter-force from chamber II is negligible because this chamber is made virtually depressurised. This is preferably achieved by the port 6) being blinded off after the piston device has been pushed up towards the position where the volume of chamber I is approximately zero. The friction in the sliding seals 15,16) is considered insignificant in this context, so that FS can be almost fully utilized to pressurize the hydraulic fluid in chamber VI. The force balance the axial forces affecting the piston device is thus given by;

1) PD*A1 = PH*A2 = FS or PH = A1/A2*PD

where PH is the absolute pressure of the hydraulic fluid

We define the accumulator unit amplification factor K as the ratio between the hydraulic pressure PH that is produced and the drive pressure PD that produces it.

Of this; K = PH/PD,

From equation 1) it appears that the amplification factor for the relevant accumulator unit is K = A1/A2. Equation 1 can thereby be transformed into;

PH= PH + PD= PD*K or 2) PH = K*PD -PAMB

where PH is the excess pressure of the hydraulic fluid in relation to the surrounding water pressure

In the following calculation example, we assume that this accumulator unit is to be installed at a depth of 500 metres, and that it must be able to operate hydraulic equipment with a nominal operating pressure of 345 bar. We want the accumulator unit to generate a hydraulic pressure of 345 bar at a depth of 500 metres.

The further calculations are simplified by atmospheric pressure being set equal to 1 bara = 1 kp/cm<2>, and by the pressure increase per 10 meters of water depth being set equal to 1 bar. In these calculations, the sealing rings' friction is considered negligible.

By inserting into equation 2) we find; 345bar = K*PAMB -PAMB

where PAMB = 51 bara . From this, K = 7.76 is found.

If an accumulator unit with an amplification factor of 7.76 is lowered to, for example, 800 msw, PD= PAMB will produce an absolute hydraulic pressure PH = 81*7.76 bara = 628.5 bara - which corresponds to 547.5 bar above PAMB. We therefore see that the prerequisite for being able to rely on this type of accumulator units at great depths is that you depend on a pressure-reducing valve that lowers the pressure of the propellant down to a level that is in some cases significantly lower than PAMB. As an example, the drive medium pressure must be lowered from PAMB = 101 bar to PD = 57.5 bar if the hydraulic pressure is still to be 345 bar (above PAMB) at 1000 msw. This is not favorable for two reasons. One is that having to lower the drive pressure significantly in relation to PAMB represents an unwanted large loss of energy, and the other is that a large pressure drop places greater stress on the pressure regulation valve, which must maintain a stable hydraulic pressure.

Operation and characteristics of a hydraulic system according to the invention

An accumulator unit according to the invention is based on the displaceable piston device being designed so that 6 chambers are preferably defined, as all these chambers - with the exception of the drive chamber I - can in principle be switched between three states where they are either depressurized, put in an open connection with the port 5) which in the following description is referred to as the inlet port, or is set in open connection with the outlet port 11) which supplies pressurized hydraulic fluid. For the further description of the invention, reference is made to fig. 3 showing an enlarged view of an accumulator unit with 6 chambers (I-VI).

The piston device 2,3,4) consists of a hollow piston rod 3) and two pistons 2,4).

The cavity is cylindrical, is arranged axially in the piston rod and cooperates with a rod 19) which is permanently mounted to the housing 1), The rod 19) has a cylindrical outgrowth 18) which is fitted with a sliding seal so that the cavity is separated leak-free into two chambers III, IV ).

In this embodiment, the piston device separates 6 separate chambers, of which 4 chambers (I, II, V, VI) have contact with pressure areas on the outside of the piston device, and 2 chambers (III, IV) have contact with pressure areas on the inside of the piston arrangement. All chambers have a separate open connection to one of the 6 ports 5-10) in the accumulator housing, and from these ports there are pipe connections to the switching device 12) which has the task of being able to produce any desired connection of the chambers.

Three of the chambers (II, III, VI) are in contact with pressure areas facing away from the inlet port 5), and pressurizing these chambers produces forces which seek to push the piston device away from the inlet port. Correspondingly, three of the chambers (I, IV, V) are in contact with pressure areas on the piston arrangement which are directed towards the inlet port 5), and pressurizing these produces forces which seek to press the piston arrangement in the direction towards the inlet port 5).

The pressure areas for the 6 chambers (I-IV) are denoted AI–AVI and are defined by the circular cross-sections A1–A5 as follows;

AI =A1, AII= A1-A2, AIII= A3, AIV= A3-A4, AV = A5-A2, AVI = A5-A4

Correspondingly, the pressure in the six chambers is denoted as PI, PII, PIII, PIV, PV and PVI.

As the forces affecting the piston arrangement in the axial direction must be in equilibrium, the following equilibrium equation can be set up based on absolute pressure;

3) PI*AI + PIV*AIV +PV*AV = PII*AII +PIII*AIII + PVI*AVI

Chamber I is the drive chamber, and will always have the pressure PD, while the other 5 chambers can in principle switch between to be depressurised, to be connected to the inlet port 5) or to be connected to the outlet port 11) for the hydraulic fluid.

There are a large number of connection options for the five chambers II-VI, but the following conditions must be met for the accumulator unit to generate hydraulic energy;

- The chamber that is in contact with the largest pressure area on the piston arrangement must function as a drive chamber and have an open connection to the drive fluid

- At least one of the chambers that is in contact with a pressure surface that is directed oppositely to the largest pressure surface on the piston arrangement must have low pressure (be virtually depressurized)

- At least one of the chambers that is in contact with a pressure surface that is directed oppositely to the largest pressure surface on the piston arrangement must have an open connection to the outlet port 11).

The operation of a gas-free accumulator unit with 6 chambers is, in principle, the same as for an accumulator with three chambers. The difference is that the establishment of six chambers provides many combination possibilities both in terms of the size of the resultant force which the pressure of the propellant produces against the piston device in the direction away from the inlet port 5) and with respect to the size and direction of the areas that utilize the transmitted power to pressurize the hydraulic fluid. The resulting modes, represented by each of the selected coupling layouts, will all be able to provide an efficient transfer of energy from the drive medium to the hydraulic fluid. The relationship between the supplied amount of propellant and the amount of hydraulic fluid that is emitted is uniquely determined by the chosen amplification factor. It is thus desirable to find a set of modes that combine large uptake of drive medium with amplification factors that enable the individual accumulator unit to be able to operate the individual accumulator unit over the widest possible depth interval. This is illustrated by table 1 and table 2, which show the properties of an accumulator unit with an embodiment in accordance with the outline of the accumulator unit shown in fig. 3. The following mutual dimensioning has been chosen;

A2 = 0.37*A1 A3 = 0.23*A1 A4 = 0.082*A1 A5= 0.54 A6 = 0.14*A1

The tables are based on only a small part of the combination possibilities as chamber II is kept depressurized in all modes, chambers III, IV and VI only alternate between being connected to the inlet port 5) or the outlet port 11), and chamber VI only alternates between being connected towards the outlet port 11) or to be depressurised. To achieve these combinations, 4 pieces are needed. two-way valves which in reality provide 24 = 16 connection options. Of these 16 connection options, 10 connection options are usable for the purpose, while the remaining 6 are not suitable for generating hydraulic power. In table 1 and table 2, we have selected the 7 connection options that are considered most suitable under the given conditions.

The heading "Volume" indicates how many liters of hydraulic fluid with PH = 345 bar can be delivered in the respective modes for an accumulator unit that has a stroke length of 100 cm and where A1 is chosen equal to 314 cm2 (corresponding to Ø=200 mm). We see that the accumulator unit can deliver 4.7 liters with the connection option which gives amplification factor =7.8. This is approximately 18% more than can be achieved with a three-chamber accumulator unit with the same amplification factor and the same piston diameter and stroke length. The reason for this increased utilization is that chambers IV and V contribute to increased utilization of the propellant.

Table 2 shows the maximum and minimum depth. The minimum depth is defined as the depth where PD = PAMB produces PH=345 bar. Greatest depth is defined as the depth where the amplification factor stated in the next column produces PH = 345 bar when PD is set equal to PAMB.

Largest ∆P indicates the pressure drop that the pressure stabilizing valve must produce at "greatest depth" to maintain PH = 345 bar.

Table 2 shows that an accumulator unit according to the given dimensioning will be able to produce a desired hydraulic pressure from typically 500 msw to just over 4000 msw by means of a switching arrangement that includes four two-way valves.

Table 1

Table 2

In Fig. 3, the mutual dimensioning in the drawings shown of accumulator units with 6 chambers is in accordance with the data given in Tables 1 and 2.

Fig. 4 a-e) shows alternative designs with respect to piston device 2-4) and rod 19) with protrusion 18). Fig. 4 a) shows a design with 4 chambers. Fig. 4b) and 4c) show designs based on 5 chambers. Figs d) and e) show designs based on 6 chambers. The choice of design is based on how the accumulator system is to function. If it is to be used in work-over systems, you will normally choose a 6-chamber solution to be able to produce the desired hydraulic pressure over a large depth interval. If the hydraulic system is to be used for the operation of a BOP, a four- or five-chamber solution may be more advantageous.

The operation of a pressure stabilizing valve according to the invention

Fig. 5 a-c show various views of a pressure stabilizing valve in a preferred embodiment.

The valve comprises a housing 24) with an inlet 41) and an outlet 29), and is designed to control the supply of drive medium to the accumulator unit's inlet port 5) by changing the flow cross-section through the valve by displacement of an axially arranged first valve body 27) in relation to an annular seat surface 28). The displacement of the valve body is produced by cooperation with a pressure-sensing element 32) which senses the difference between the hydraulic fluid pressure and the ambient pressure PAMB. This is achieved by establishing pipe connections between the outlet port 11) and port 34) in the valve housing, and a corresponding connection between a line connected to the reservoir 14) and port 30).

In fig. 5a), the pressure stabilizing valve is held in the closed position by the fact that the first valve body 27) is pressed against the seat 28). This valve body is displaceable in a narrow guide in a sleeve 25) which is arranged axially in the housing 24). The first valve body 27) thereby forms a displaceable end wall in chamber VII. This chamber is supplied with drive fluid from the inlet 41) via a narrow channel 40), and can deliver drive fluid to the outlet 29) by a second axially arranged valve body 36) being displaced away from a seat 37) which is arranged in the first valve body 27). This second valve body 36) is displaceable only a short distance away from the seat 37) before it hits the inside of the first valve body, so that a further displacement in the direction away from the seat 37) will cause the first valve body to be displaced away from the seat surface 28) and opens for the flow of liquid. When the second valve body 36) lies close to the seat 37), the pressure in chamber VII will be PAMB, and the first valve body 27) is pressed down against the seat surface 28) by the tension from a spring 39) and by the pressure difference between the inlet 41) and the outlet 29) . As the second valve body will typically have a diameter of 40 mm, a considerable force will initially be required to push the first valve body away from the seat 28). By pushing the second valve body away from the seat 37), it is opened for the discharge of liquid from chamber VII to the outlet 29). The narrow channel 40) can only provide modest refilling, and thereby the force which seeks to press the first valve body against the seat surface 28) can typically be reduced by 90%. This leads to the regulatory process becoming significantly more precise.

The pressure stabilizing valve is provided with a spring pack 31) which defines the desired hydraulic pressure in relation to the ambient pressure. It can typically be dimensioned so that the pressure-sensing element is displaced 13 mm in the direction of the inlet 41) if sensed pressure falls 30 bar in relation to a desired pressure, as the first valve body can typically be dimensioned so that such displacement changes the flow cross-section from zero to an opening corresponding to Ø35. For example, you can choose for the pressure stabilizing valve to go to the fully open position if the hydraulic pressure drops to 320 bar, and go to the fully closed position when the consumption drops so that the hydraulic pressure rises to 350 bar.

The valve device comprises a first 33), a second 35) and a third 26) sliding seal. The first sliding seal 33) is designed to prevent leakage between the pressure sensing chamber VIII and chamber IX which holds the pressure PAMB.

The second sliding seal 35) is designed to prevent leakage between the pressure sensing chamber VIII and the outlet 29).

The third sliding seal 26) is designed to limit unwanted leakage into chamber VII.

Maintenance of hydraulic capacity

A hydraulic system according to the invention will normally be based on the use of a non-lubricating liquid which is used both as a driving medium and as a hydraulic fluid, this liquid being recycled in a closed circuit. In a preferred embodiment, the hydraulic system is permanently pressurized, so that the relevant hydraulic equipment can be activated by only opening a valve which is arranged between this equipment and the outlet port 11). Consequently, it is desirable that consumed hydraulic capacity should be able to be recovered/maintained while the accumulator system is pressurized, and that the recovery process has the least possible effect on the hydraulic pressure that is generated.

A charging system according to the invention is based on hydraulic capacity being recovered by a collaboration between two types of pump which are both commercially available; An electrically driven subsea pump 22) which is designed to be able to increase the pressure in non-lubricating fluids and an intensifier pump 42). The latter is preferably based on Norwegian patent 344418. The charging process is based on a position indicator, not shown, which is designed to activate the charging system when one or more piston devices have been moved a given distance as a result of some hydraulic capacity being consumed. The charging system must be deactivated when the indicators signal that the hydraulic system has full capacity again. This type of function is considered known technology. Activating the charging system means that the electrically driven pump starts to suck liquid from the drive chamber via the pipe that connects the pressure stabilizing valve 20) with the inlet port 5), at the same time that the intensifier pump starts to push liquid back to the hydraulic chambers of the accumulator units via the outlet port 11). Only a modest increase in the pressure at the outlet port 11) will be required for liquid to be pushed back into the accumulator chambers, then under the assumption that the amount of liquid pumped out from the accumulator unit's drive chambers is sufficiently large in relation to the amount of liquid that is sought to be pressed into the hydraulic chambers. This is illustrated by the following calculation example;

The example assumes that the accumulator units will generate hydraulic power at a depth of 600 metres, and that they are configured with an amplification factor of 7.8 (cf. table 2). In this situation, it is chamber III that acts as a hydraulic chamber. When the amplification factor is set to 7.8, it means that 7.8 liters of fluid must be removed from the drive chamber for every liter of fluid that is forced back into chamber III.

The charging function is based on the liquid that the electrically driven pump removes from the drive chamber being pressurized to a sufficient extent so that it can function as a drive medium for the intensifier pump, as we choose to base the charging system on an intensifier pump 42) with an amplification factor of 8. This means that 8 liters liquid that is removed from the drive chamber will be able to be used by the intensifier pump to increase the pressure of 1 liter of liquid that is taken up from the reservoir 14) via the inlet 46) to a sufficient degree that this liquid can be forced into chamber III via the outlet port 11). Under these conditions, 0.2 liters of liquid will be removed more than what is needed for chamber III to receive 1 liter of liquid. Consequently, the pressure in the drive chamber will drop so that the pressure regulating valve opens and compensates for this volume.

A relevant type of electrically driven pump can typically have the capacity to pump 40 liters of liquid per minute. An intensifier pump with an amplification factor of 8 will then be able to pump 5 liters of liquid per minute back to the accumulator units. We assume as an example that the pressure stabilizing valve is designed to maintain a hydraulic pressure of 345 bar. We further assume that the intensifier pump must generate a liquid pressure that is 15 bar higher – i.e. 360 bar – in order to force liquid back to the accumulator units.

As the intensifier pump has an amplification factor of 8, it will then require a drive pressure of approximately 360 bar/8 = 45 bar (above ambient water pressure). Based on table 2, the pressure compensating valve under the given conditions will be designed to create a pressure drop ∆P = 18.7 bar*(61-51)/(86.9-51) = 5.2 bar. This means that the electrically driven pump will increase the pressure on the liquid that is sucked out of the drive chamber from approx. PAMB – 5.2 bar to PAMB+ 45 bar. The charging system is self-regulating in the sense that it itself regulates the pressure entering the intensifier pump inlet 44) to the level required to charge the accumulator units.

We then imagine that the hydraulic system will be charged while it is placed at a depth of 1,500 metres. Based on table 2, the accumulator unit must then be configured for an amplification factor of 3.76. When the charging system is activated, the electrically driven pump will start pumping 40 liters per minute from the drive chambers, with the result that the intensifier pump charges the accumulator units at 5 liters per minute. In this situation, the amount that must be removed from the drive chambers will be 5 litres/min.* 3.76 = 18.8 litres/min. As the electrically driven pump removes 40 litres/minute, the pressure stabilizing valve in this situation will compensate for this by to provide liquid supply to the drive chambers of size 21.2 litres/min.

Effect of leakage in sliding seals

A hydraulic system according to the invention will have a certain amount of fluid leakage into the chambers, which are depressurized via the various sliding seals. This leakage will normally be small, and not affect the hydraulic system's capacity or function to a significant extent unless the system is kept pressurized for a long time. However, it may be desirable to be able to remove liquid from these chambers. Figure 6 indicates that the accumulator units are oriented so that any liquid in the virtually pressureless chambers is given the opportunity to flow down to a low-lying tank 23). This tank can be supplied with a built-in, single-acting pump which can be activated by opening the valve 47) and pump received liquid back to the reservoir 14). This can be solved in various ways using known techniques and will not be described in more detail.

Claims (3)

Claim 1. An underwater-based, depth-compensable accumulator system which consists of at least one accumulator unit in cooperation with a switching device (12), the accumulator unit comprising an axially displaceable piston device (2-4) which delimits a number of chambers (I-VI) and which is arranged to produce a desired pressure on hydraulic fluid that is stored in the accumulator unit by utilizing a supplied propellant's excess pressure in relation to one or more virtually pressureless chambers in the accumulator unit, characterized by - the accumulator unit consists of an accumulator housing (1) with leak-free cylindrical guides for the piston device (2-4) which comprises a hollow piston rod (3) and two pistons (2,4) which cooperate with a rod (19) which is permanently mounted in the accumulator housing (1) and which has a cylindrical outgrowth (18) which is fitted with a sliding seal whereby up to 6 separate chambers (I-VI) are delimited in the accumulator housing with separate connections to ports (5-10) which are arranged in the accumulator housing, with three of the chambers (II, III, VI) are in contact with pressure surfaces on the piston arrangement facing the inlet port (5) so that pressurization of these chambers produces forces that seek to press the temple device in the direction of the inlet port (5), and the other chambers (I, IV ,V) has contact with pressure surfaces facing away from the inlet port (5) so that pressurizing these chambers produces forces that seek to press the piston arrangement in a direction away from the inlet port (5), - the piston device's largest pressure surface faces chamber I, which during operation is permanently connected to pressurized propellant via the inlet port (5), - at least one of the up to three separate chambers (II, III, VI) that is in contact with a pressure surface which is directed oppositely to the largest pressure surface of the piston device, has been made virtually depressurised, - the switching device (12) has an outlet port (11) for releasing hydraulic fluid, and comprises solenoid-controlled, hydraulic or mechanically operated switching valves which are connected via the ports (5-10) in the accumulator housing (1) to each of the chambers (I- VI) so that it is possible to carry out a desired change in terms of which of the chambers are to be depressurized and which are to be connected to the the inlet port (5) or towards the outlet port (11) in order to thus be able to change the relationship between the pressure in the supplied driving medium and the pressure on the hydraulic fluid which is released via the outlet port (11) - at least one of the chambers that is in contact with a pressure surface that is directed opposite to the piston device's largest pressure surface has an open connection with the outlet port (11), 2. An underwater-based accumulator system according to claim 1, characterized in that the drive medium is supplied from a flexible reservoir (14) that contains liquid with approximately ambient pressure, and that a pressure stabilizing valve (23) is arranged between the outlet from this reservoir and the inlet port (5) which senses the pressure at the outlet port (11) of the switching device (12) and which is designed to regulate the supply of drive medium to the accumulator units so that the hydraulic pressure at the outlet port (11) is maintained at a fixed, preset level 3. An underwater-based accumulator system according to claims 1 and 2, is characterized in that the pressure stabilizing valve (23) is designed to control the supply of propellant to the accumulator units by displacing an axially arranged first valve body (27) in relation to an annular seat surface (28) and thus varying the flow cross-section for propellant which is led from an inlet (42) to an outlet (29) in the valve housing (24), the displacement of the valve body being produced in cooperation with a pressure-sensing element (32) which senses the difference between the hydraulic fluid pressure at the outlet port (11) and the surrounding water pressure, and which is biased in the direction towards the inlet (42) of a spring (31) so that the pressure-sensing element (32) has a defined position when sensed hydraulic pressure corresponds to the desired level, the valve arrangement also comprising; - a sleeve such as (25) which is arranged axially in the valve housing (1) and which has a narrow internal guide for the first valve body (27) so that a chamber (VII) appears in which this valve body forms a displaceable end wall, this chamber is supplied with fluid from the inlet (42) via a narrow channel (41) which is arranged in the sleeve (25) - a second valve body (36) which is limited axially displaceable in relation to a seat (37) which is arranged in the first valve body (27), so that a displacement of the second valve body (36) away from the seat (37) produces a leakage of liquid from chamber VII which causes the pressure in this chamber to drop to a fraction of the pressure at the inlet (42), and so that a further displacement of the second valve body (36) in the same direction will displace the first valve body (27) in same direction - a first (33) and a second (35), and a third (26) sliding seal, there - the first sliding seal (33) is arranged to minimize leakage between the pressure sensing chamber VIII and chamber IX, - the second sliding seal (35) is designed to prevent leakage between the pressure sensing chamber VIII and the outlet (29) - the third sliding seal (26) is designed to limit unwanted leakage into chamber VII.